Here's why:

* Venus's atmosphere is 96.5% carbon dioxide. This creates a very strong greenhouse effect, trapping heat and making Venus the hottest planet in our solar system.

* The atmospheric pressure at the surface of Venus is 92 times that of Earth's. This is equivalent to being about 900 meters (3,000 feet) underwater on Earth.

While Mars has an atmosphere, it's much thinner and composed primarily of carbon dioxide. Mercury has a very thin exosphere, which is barely considered an atmosphere. Earth's atmosphere is significant but is much thinner than Venus's.